In recent years, there has heen increasing interest in digital TV due to the greater availability of low cost digital hardware and memories. The digital TV allows the user to reformat the incoming video signals. For example, in a digital TV provided with a pix-in-pix feature, an auxiliary or secondary video signal SVS (e.q., from VCR tuner and IF circuits) defines a small picture on the display screen as an inset within a full picture defined by a main or primary video siqnal PVS (e.g., from TV tuner and IF circuits).
When either the primary or the secondary video signal is a nonstandard signal, timing or skew errors can occur in the displayed inset picture, which are manifested as jagged vertical edges. As used herein, a nonstandard signal is a video signal having a non-standard number of color subcarrier cycles in a horizontal period, and usually a non-constant or jittering horizontal period. Typical examples of non-standard signals are video signals generated by a video cassette recorder (VCR) or a video disc player (VDP). The cyclical variations in the horizontal line period (also known as time base errors) are typically caused by variations in the tape or disc speed.
The skew errors not only arise when one (or both) of the incoming video signals is (are) non-standard, but they can also arise when two standard video signals are received from two sources which are not synchronized with each other. This is so because the synchronizing signals, while within the tolerance limits of a particular signal standard (e.g., NTSC), have different frequencies, whereby the synchronizing signals precess in phase with respect to each other and can cause skew errors.
To understand how these timing or skew errors may affect the overlaid inset image, it is useful to know how the primary and secondary video signals PVS and SVS are processed in a typical pix-in-pix TV receiver. The secondary video signal SVS is sampled and digitized at instants determined by a sampling clock signal. The digital samples representing the secondary video signal SVS are then subsampled, both horizontally and vertically, to develop a stream of samples which represent a reduced size image. For example, for a 3-to-1 reduction in the picture size, every third sample and every third line is saved, and the intermediate samples and lines are discarded.
The digital samples taken during one field of the secondary video signal SVS are stored in a field memory. These samples are read out from the memory usinq a clock signal which is desirably related to the display deflection signal (e.g., horizontal synchronizing component of the primary video signal PVS). The samples read out from the memory are converted into an analog signal SVS' representative of the reduced-size secondary picture. A video switch, having input terminals coupled to receive the primary video signal PVS and reduced-size secondary video signals SVS', applies an appropriate one of the two input signals to a display device at each instant to produce a small picture within a large picture. U.S. Pat. No. 4,638,360 (Christopher et al.) entitled "TIMING CORRECTION FOR A PICTURE-IN-PICTURE TELEVISION SYSTEM" describes an illustrative pix-in-pix TV receiver, and a skew error correction system therefor.
The afore-mentioned Christopher et al. patent shows two embodiments of the pix-in-pix TV receiver. In one embodiment (FIG. 2), the sampling clock signal is locked to the color burst component of the secondary video signal SVS. In the second embodiment (FIG. 3), the sampling clock signal is locked to the color burst component of the primary video signal PVS. In both embodiments, the digital samples are corrected for skew errors: (1) before they are stored in the memory (to correct for skew errors caused by the variations in the horizontal line period of the secondary video signal SVS), and (2) after they are read out from the memory (to correct for skew errors caused by the variations in the horizontal line period of the primary video signal PVS).